I made sure that my water level was as high as possible. I have a glass cover on 80% of the tank. I dropped the HOB pump to its lowest setting. I continue to have about 1-2 bubbles per second into my Hagen ladder. The bubbles usually decrease in diameter from about 4 to 1 mm. I have a brand new pH test kit and KH kit.
I was eventually able to see the pH drop from about 7.6 to 7.0. The pH drop did not occur until several days after I did a 40% water change. And even though I continued to add lots of gas, the pH rose back up again after about a week.
I did another large (40%) change about 2 weeks later. Once again, after a few days, the pH dropped from 7.6 to about 7.0.
My layperson's interpretation is that there is something in the tank (gravel?) that is buffering against a pH change. The influx of a good portion of new tap water temporarily removes some of the buffering capacity (KH and GH are about 100 ppm after the water change). This is why the gas is able to lower the pH significantly a day or two after the water change. But once the water has been in the tank for some time, the addition of CO2 doesn't produce a pH change.
I could be entirely wrong!
But at the end of the day, the plants are growing well, and I am getting constant streams of little gas bubbles off some of the amazon swords.